1. Technical Field
The present invention relates to mixed-signal circuits in general, and in particular to Delta-Sigma modulators.
2. Description of Related Art
Delta-Sigma analog-to-digital converters (ADCs) are capable of shaping quantization noise spectrums in the frequency domain such that more quantization noise can be placed in out-of-band frequencies while less quantization noise will remain in the pass band of interest. Thus, Delta-Sigma ADCs are commonly known for their high-resolution properties.
A Delta-Sigma ADC includes a feedback path containing feedback signals to be subtracted by analog signals from an analog input in order to generate error signals for a loop-filter. Thus, the Delta-Sigma ADC also requires a feedback digital-to-analog converter (DAC) to convert digital signals to corresponding analog signals for the feedback path. Typically, multiple analog unit-elements, such as unit-sized capacitors or unit-sized current sources, are employed within the feedback DAC. Any mismatch in the analog unit-elements, however, can cause higher pass band noise or distortion of desired signals, which directly affects the overall performance of the Delta-Sigma ADC. In order to alleviate the analog unit-element mismatch sensitivity, a dynamic element matching (DEM) technique can be introduced to boost the Delta-Sigma ADC's noise and linearity performance within the pass band of interest.
DEM, which is typically implemented as a DEM algorithm in conjunction with a DEM shuffler, operates to shuffle analog unit-elements in a feedback DAC. For example, a DEM algorithm may use a feedback DAC's input samples to generate control signals for a DEM shuffler. The DEM shuffler then uses the control signals to shuffle the analog unit-elements within the feedback DAC. Generally speaking, a DEM algorithm aims to shape analog unit-element mismatch energy in the frequency domain so that most of the analog unit-element mismatch energy can be placed in out-of-band frequencies where the mismatch energy can be subsequently filtered out without affecting any desired signals in the pass band of interest. With lesser analog unit-element mismatch energy remaining in the pass band of interest, the noise floor or linearity of the Delta-Sigma ADC will no longer be limited by the feedback DAC's mismatch energy within the Delta-Sigma ADC.
A commonly employed DEM algorithm is the data weighted averaging (DWA) algorithm. The DWA algorithm uses analog unit-elements sequentially in a round-robin fashion, iterated based on the digital output value of a quantizer, such that each of the analog unit-elements is used exactly once before it is used again. In the frequency domain, the DWA algorithm basically applies a first-order high-pass filter to a feedback DAC's analog unit-element mismatch energy. For a low-pass Delta-Sigma ADC where the pass band of interest is around DC, applying a first-order high-pass filter to a feedback DAC's analog unit-element mismatch energy will move the analog unit-element mismatch energy away from the pass band of the low-pass Delta-Sigma ADC. As a result, a higher linearity and lower noise floor can be achieved in the pass band of the low-pass Delta-Sigma ADC.